The invention relates to the field of papermaking. More in particular, the invention relates to the use of a novel dry strength agent in the wet-end of the papermaking process.
Traditionally, cationic starches are applied as dry strength agents in the wet-end of the paper production process. Due to the presence of anionic groups on the cellulose fibres and fillers, cationic starch binds to the fibres and fillers. This electrostatic interaction also gives an improvement in the retention on the sieve of both the cellulose fibres and the fillers in the paper sheet. Beside as dry strength agent and retention support cationic wet-end starches are also used for alkenyl succinic anhydride (ASA) emulsification in the wet-end.
A serious drawback of the use of cationic starch is its limitation of the amount of cationic starch that can be used. Addition of cationic starch to the fibres gives rise to the neutralisation of the anionic charge on the cellulose fibres and fillers and eventually overcharging leading to an overall cationic charge. This has to be avoided because overcharging results in a dramatic reduction in wet-end performance, overall retention and formation, of the paper machine.
In the paper industry there is an increasing demand for dry strength. This demand is a result of the following trends: use of more cheap and/or secondary cellulose fibres, the increase in filler content in the paper sheet and the use of a pre-metering size press. Therefore, there exists an increasing need for new wet-end starches which allows for increasing the addition levels in the wet-end without the risk of overcharging the cellulose fibres and fillers.
In accordance with the invention it has surprisingly been found that the use of a hydrophobic starch as dry strength agent avoids neutralisation of the anionic charges on both cellulose fibres and fillers, while having a strong binding affinity to the cellulose fibres and fillers, thereby providing the required contribution to paper strength.
The use of a hydrophobic starch as dry strength agent does not have any substantial influence on the overall charge balance in the wet-end of the papermaking process. It can therefore be used in higher amounts than conventional dry strength agents without disturbing the wet-end performance, overall retention and formation at the paper machine.
Hydrophobic groups have a low affinity for an aqueous environment. When added to water, hydrophobic groups show a strong tendency to avoid contact with water molecules. In the presence of solid particles, like cellulose fibres and the filler materials used in papermaking, it has been found that the hydrophobic starch tends to adsorb to these particles, rather than staying in the aqueous phase. Without wishing to be bound by theory, it is postulated that this behaviour explains the binding capacity and performance of the hydrophobic starch as dry strength agent in the wet-end of papermaking.
The international patent application WO 99/55964 discloses a process for the production of paper from a suspension containing cellulosic fibres which comprises adding to the suspension a drainage and retention aid comprising a cationic or amphoteric polysaccharide, forming and dewatering the suspension on a wire, wherein the cationic polysaccharide has a hydrophobic group. The degree of substitution (DS) of anionic groups for the polysaccharide is from 0 to 0.2. However, the polysaccharide is also substituted with cationic groups and the DS of cationic groups is from 0.01 to 0.5, preferably from 0.025 to 0.2. The DS of cationic groups is always higher than that of anionic groups, making these polysaccharides overall cationically charged. Therefore, the binding mechanism to the fibres is still in accordance with the charge interaction mechanism.
The international patent application WO 2004/031478 discloses a cationised polysaccharide product comprising a polysaccharide having at least one first substituent having an aromatic group and at least one second substituent having no aromatic group, wherein the first substituent and the second substituent are present in a molar ratio in the range of 10:1 to 1:10. Also disclosed is a process for making paper wherein the cationised polysaccharide is added to an aqueous suspension containing cellulosic fibres.
The hydrophobicity of aliphatic groups is dependent on the number of carbon atoms. Compared to aromatic groups with the same number of carbon atoms, the aliphatic carbon chain is more hydrophobic. In accordance with the invention it has surprisingly been found that hydrophobic anionic starches bearing an aliphatic carbon chain, with an overall negative charge density between 0 and −0.09 μeq/mg exhibit a high affinity for solid particles in the wet-end. Thus, in accordance with the invention there is a preference for hydrophobic starches having an overall negative charge density between 0 and −0.09 μeq/mg, and a greater preference for such starches having an overall negative charge density between −0.005 and −0.07 μeq/mg.
The dry strength agent in accordance with the invention is a hydrophobic starch which may, in principle, be derived from any botanical source. Both root or tuber starches, such as cassava or potato starch, and cereal and fruit starches, such as maize, rice, wheat or barley starches can be used. Legume starches, such as pea or bean starches, can also be used. In a preferred embodiment, the starch is a root or tuber starch, more preferably potato or cassava starch.
Natural starches typically have a more or less fixed ratio of the two components of starch, amylose and amylopectin. Of some starches, such as maize or rice starch, a natural occurring variety exists which contains essentially only amylopectin. These starches, which are normally called waxy starches, can also be used. Of other starches, such as potato or cassava starch, there are genetically modified or mutant varieties, which also essentially only contain amylopectin. It will be understood that the use of these varieties, typically comprising more than 80 wt. %, preferably more than 95 wt. %, based on dry weight of the starch, of amylopectin, is also within the scope of the invention. Finally, also starch varieties that are high in amylose, such as high amylose potato starch, can be used for the preparation of a dry strength agent according to the invention. In accordance with the invention, starches of all amylose to amylopectin ratios may be used. However, it is preferred that a starch is used having a regular or increased amylopectin content.
The starch for making a hydrophobic starch in accordance with the invention is preferably a native starch. However, if desired, the molecular weight of the starch may be decreased or increased by any method known in the art, such as acidic degradation or oxidation, prior to or simultaneous with the introduction of the hydrophobic group.